Free Sialic Acid Quantification, Urine

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Condition Description

Sialic acid storage diseases (SSDs) are severe autosomal recessive neurodegenerative disorders caused by a transport defect across the lysosomal membrane, which leads to accumulation of sialic acid in tissues, fibroblasts, and urine. Infantile free sialic acid storage disease (ISSD) is the most severe form of this disorder. Babies with this condition have severe developmental delay, hypotonia, and failure to thrive. They may have coarse facial features, seizures, bone malformations, hepatosplenomegaly and cardiomegaly. The abdomen may be swollen due to the enlarged organs and an abnormal buildup of fluid in the abdominal cavity (ascites). Affected infants may have a condition called hydrops fetalis in which excess fluid accumulates in the body before birth. Children with this severe form of the condition usually live only into early childhood.

Mutations in the SLC17A5 gene cause all forms of sialic acid storage disease. This gene provides instructions for producing a protein called sialin that is located mainly on the membranes of lysosomes, compartments in the cell that digest and recycle materials. Sialin moves free sialic acid, produced when glycoproteins or glycolipids are broken down, out of the lysosomes to other parts of the cell. Free sialic acid means that the sialic acid is not attached (bound) to other molecules. Elevated free sialic acid in urine is a useful first tier screening test for sialic acid storage disorders. Some cases may present with normal urine levels. In these individuals, determination of CSF sialic acid levels may be necessary.

In addition to sialic acid transporter disorder, urine free sialic acid levels could be elevated in a patient with sialuria, a rare autosomal dominant condition due to certain mutations in GNE gene.

Indications

This test may be indicated for:
  • Patients with severe developmental delay, coarse facial features, hepatosplenomegaly, cardiomegaly, prenatal or neonatal nonimmune hydrops, deterioration or loss of milestones in infancy, seizures, proteinuria, nephrotic syndrome, skeletal features, and hypotonia.  
  • Older patients with spasticity, epileptic seizures, and athetosis, abnormal myelination and white matter changes.  
This test is not recommended for carrier screening.  

Methodology

HPLC-ESI-MS/MS.

Free N-acetylneuraminic acid (Neu5Ac or NANA) is measured as the representative of sialic acid. 

Reference Range

     

Specimen Requirements

Additional Specimen Collection/Handling Instructions Required for this Test
Rejection criteria:
- Urine that has been left at room temperature for more than 7 days will be rejected.
- Extremely dilute urine with a creatinine value less than or equal to 0.02 mg/mL may be rejected at the discretion of the lab director.

Submit only 1 of the following specimen types

Type: Urine

Specimen Requirements:

2-5 mL urine, random or first morning void preferred. No preservative. Ship frozen.

Specimen Collection and Shipping: Ship frozen sample on dry ice with overnight delivery.

Special Instructions

Clinical information is required for a complete interpretation.
  • SSLC1: Free Sialic Acid Storage Disorders: SLC17A5 Gene Sequencing.
  • SGNEX: Congenital Disorder of Glycosylation, GNE-related: GNE Gene Sequencing.
  • DGNEX: Congenital Disorder of Glycosylation, GNE-related: GNE Gene Deletion/Duplication.
  • Custom Diagnostic Mutation Analysis (test code KM) - is available to individuals.  Please contact the laboratory genetic counselor to arrange testing. 
  • Prenatal testing may be available by molecular methods.  Please contact the laboratory genetic counselor to determine the availability of prenatal testing. 

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